Capillary electrophoresis is a technique of considerable interest in the analysis of biological mixtures as it provides a number of distinct advantages over other separation processes. One advantage of capillary electrophoresis is the small volume of the capillary tube interior. This permits one to perform separations on extremely small volumes, and at high speeds. Another advantage of capillary electrophoresis is the rapid rate at which heat is dissipated outward from the capillary tube due to the capillary's narrow bore. This permits the use of a high voltage to drive the electrophoresis which, in turn, provides for separations at high speed and with high efficiency. Each of these advantages renders capillary electrophoresis particularly useful for analyzing samples of biological interest, particularly mixtures of peptides, proteins, and nucleic acids.
One of the problems with capillary electrophoresis, however, is that frequently the sample to be analyzed is very dilute. As with other methods of analysis, there is a detection limit under which one cannot go if accurate analytical data are required. A concentration step, therefore, is often required prior to starting the method of analysis. Frequently, however, this concentration step cannot be carried out without a large loss of sample, particularly when the same volume is 1-10 .mu.L or smaller. Accordingly, there is a need for efficient concentration techniques which are applicable to small-volume samples. Additionally, there is a need for concentration techniques which can be used in free electrophoresis for high-mobility proteins and peptides. These and other problems are addressed by the present invention.